1. Field of the Invention
The following relates to a method and apparatus for homogeneously magnetizing an exchange-coupled layer system of a digital magnetic memory cell device comprising an AAF layer system and an antiferromagnetic layer that exchange-couples a layer of the AAF layer system.
2. Background Information
A digital magnetic memory cell device stores information on a magnetic basis. An individual memory cell device is generally part of a memory device, often also called an MRAM (magnetic random access memory). Such a memory can carry out read and/or write operations. Each individual memory cell device comprises a soft-magnetic read and/or write layer system, separated by means of an intermediate layer from a hard-magnetic reference layer system, which is formed as an AAF system in the case of the present type of memory cell device. The magnetization of the reference layer of the reference layer system is stable and does not change in an applied field, while the magnetization of the soft-magnetic read and/or write layer system can be switched by means of an applied field. The two magnetic layer systems may be magnetized in a parallel or antiparallel fashion with respect to one another. The two aforementioned states in each case represent a bit of information, i.e. the logic zero (“0”) or one (“1”) state. If the relative orientation of the magnetization of the two layers changes from parallel to antiparallel, or vice versa, then the magnetoresistance changes by a few percent over this layer structure. This change in the resistance can be used for reading out digital information stored in the memory cell.
The change in the cell resistance can be identified by a voltage change. By way of example, in the case of a voltage increase, the cell may be occupied by a logic zero (“0”) and, in the event of a voltage decrease, the cell may be occupied by a logic one (“1”). Particularly large changes in resistance in the region of a few percent have been observed when the magnetization orientation changes from parallel to antiparallel, and vice versa, in cell structures of the GMR type (giant magnetoresistance) or TMR type (tunnel magnetoresistance).
An important advantage of such magnetic memory cells is that the information is stored persistently in this way, and is stored without maintaining any basic supply even with the device switched off, and is immediately available again after the device is switched on, in contrast to known conventional semiconductor memories.
A central component part in this case is the reference layer system, formed as an AAF system (AAF=artificial antiferromagnetic). Such an AAF system is advantageous on account of its high magnetic rigidity and the relatively low coupling to the read and/or write layer system by virtue of the so-called orange peel effect and/or by virtue of macroscopic magnetostatic coupling fields. An AAF system generally comprises a first magnetic layer or a magnetic layer system, an antiferromagnetic coupling layer and a second magnetic layer or a magnetic layer system which is coupled with its magnetization via the antiferromagnetic coupling layer oppositely to the magnetization of the lower magnetic layer. Such an AAF system may be formed e.g. from two magnetic Co layers and an antiferromagnetic coupling layer made of Cu.
In order to improve the rigidity of the AAF system, that is to say its resistance toward external fields, it is customary to arrange an antiferromagnetic layer at that magnetic layer of the AAF system which is remote from the read and/or write layer system. This antiferromagnetic layer additionally pins the directly adjacent magnetic layer in its magnetization, with the result that the AAF system overall becomes harder (exchange pinning or exchange biasing).
The magnetic rigidity of the AAF system corresponds to the amplitude of the applied external field which is required for rotating the magnetizations of the two ferromagnetic layers in the same direction, that is to say for parallel setting. This limits the magnetic window for read and write applications of such a memory cell device.
In this case it is always an aim for the magnetic layers of the AAF layer system, which, by way of example, besides the material combination mentioned in the introduction, may also comprise two ferromagnetic CoFe layers and an Ru layer introduced in between, to be magnetized as homogeneously as possible, ideally with a single homogeneous magnetization direction. The magnetization in the case of the memory element in question having the AAF layer system and the additional exchange-coupling or pinning antiferromagnetic coupling layer, e.g. made of IrMn, is effected in such a way that, after the production of the layer stack, the layer stack is heated to a temperature greater than the blocking temperature of the antiferromagnetic layer, that is to say e.g. of the IrMn, a strong magnetic field that saturates the two magnetic layers of the AAF layer system being present during this. This leads to an orientation of the magnetic layer magnetizations, and, on account of the couplings, also of the magnetization of the antiferromagnetic layer. The temperature is subsequently decreased again. If the external setting magnetic field is then likewise withdrawn, the magnetization of the magnetic layer which is not coupled to the antiferromagnetic layer begins to rotate on account of the AAF system coupling.
In this case, however, a multiplicity of so-called 360° walls form in the magnetization of the layer. These 360° walls, which are manifested as winding, sinuous lines in the context of carrying out a domain observation, entail a series of disadvantages. Thus, by way of example, the signal that can be tapped off via the memory element, for example a TMR signal in the case of a TMR memory element (TMR=tunnel magnetoresistive), is reduced. The magnetization reversal behavior of the measurement layer, e.g. made of permalloy, which is separated from the AAF layer system by means of a decoupling layer, e.g. made of Al2O3, is also less favorable on account of the leakage fields, those via the 360° walls, in which the magnetization rotates once through 360°.